The overall goal of this proposal is to study the biochemical mechanisms involved in the regulation of gene expression during embryonic growth, particularly during myogenesis and limb bud differentiation. Although eukaryotic gene expression is primarily regulated at the transcriptional level, translational controls involving a variety of mRNAs, particularly in very early stages of embryonic development are known to operate. We have recently isolated and characterized a novel cytoplasmic translation inhibitory 10S ribonucleoprotein (iRNP) containing a 4S heterogeneous RNA (iRNA) species from 13-14 day old chick embryonic muscle. The iRNP are potent inhibitors of mRNA translation in vitro. Their site of action is the blocking of mRNA binding to ribosomes during the initiation phase of peptide synthesis. We have recently cloned and sequenced one iRNA subspecies which shows considerable "anti-sense" complementarity to a highly conserved domain of 22S rRNA, a component of the translation machinery. These results suggest that iRNA mediated translational inhibition may act as a novel mechanism of controlling cellular mRNA levels in embryonic muscle. The objectives of this project are: to probe whether iRNA and iRNP-like "regulators" function in limb bud and somites; to further characterize by molecular cloning the iRNA of embryonic muscle with respect to resolution and characterization of subspecies; delineation of their sequences; quantitation of their levels as a function of embryonic growth; to define the mechanism of their biological activities, and to isolate and characterize their genes. The proposed studies will increase our understanding of the structure of iRNA and iRNP and their role in the regulation of gene expression during embryonic development.